Various configurations of gas turbine engines include a forwardly projecting engine shaft about which an air inlet duct is positioned. For example, a turboprop engine and installation may have an upwardly offset reduction gear boxes that drives the propeller and downwardly offset air intake ducts. Further a turboprop may have a coaxial reduction gear box with a bifurcated air inlet duct with an upper and a lower air inlet duct and intake orifice. Especially in the case of aircraft engines, the size and weight of the inlet duct and the effect that the duct geometry has on the size and weight of the engine shaft must be minimized.
The conflict between the air inlet duct and a forwardly projecting shaft has conventionally been resolved by adopting either a single bottom inlet or a bifurcated inlet having two inlets top and bottom. In contrast, when the gas turbine engine is configured as a turbofan, the air inlet and fan casing draw in air in a generally axial direction. In the case of a single inlet, the air inlet duct is diverted downwardly and thereby allows the use of an upwardly offset reduction gearbox. In order to provide a relatively large curvature and minimize aerodynamic losses in airflow in advance of the compressor stage, the single inlet design generally makes the engine longer with associated penalties in shaft length, engine weight and larger outer surface area. A benefit is that a single bottom inlet frees the top of the reduction gearbox for use as an engine mounting surface.
A second conventional design choice is to bifurcate the inlet duct to have two separate branch inlets usually top and bottom. A bifurcated duct results in a shorter engine but which restricts the engine mounting surface locations. Where a bifurcated top is located on the top of the engine, the mounting location possible on the top of the engine must be shifted aftwards to clear the duct. As a result, the engine casing structure must be reinforced to accommodate the overhanging weight and therefore weight and size penalties also apply to the engine when a bifurcated inlet duct is used.
Features of the invention will be apparent from review of the disclosure, drawings and description of the invention below.